1. Field of the Invention
The present invention relates to a permanent magnet pump motor having a permanent magnet rotating electric machine that includes a rotor and a stator, and a permanent magnet pump motor system incorporating the same.
2. Description of the Related Art
Unlike ordinary pump systems, LNG pump motors have an integrated structure in which a rotor, a stator, an impeller, and a diffuser are built into a pump casing. Such a LNG pump motor is operated in an LNG fluid having a temperature of −162° C. and thus there arise problems inherent in low temperatures.
Rare-earth magnets, such as neodymium magnets and praseodymium magnets, in particular, have a property that the permanent magnet contracts in a magnetization direction and expands in a direction perpendicular to the magnetization direction at low temperatures.
This requires that linear expansion of the electrical steel sheet and the permanent magnet be taken into consideration for the shape of a magnet insertion hole in the electrical steel sheet in which the magnet is to be inserted.
In addition, since the motor is operated with the LNG packed therein, loss caused by fluid friction occupies a good part of loss of the motor. Making a rotor compact is effective in reducing the fluid friction.
Under the circumstances, various forms of permanent magnet rotor structures are being examined relative to electrical characteristics of the motor and the magnet insertion hole in the rotor, as disclosed in JP-2006-311730-A.